Integrated hyperspectral remote sensing, geochemical and isotopic studies for understanding hydrocarbon-induced rock alterations

The main objective of this work was to determine if there are characteristic mineral assemblages and chemical changes in areas affected by hydrocarbon microseepages. For this purpose remote sensing was utilized for mapping surficial rock alterations, and geochemical tools were used to understand the alteration processes. The key area chosen for this type of work were altered and unaltered Wingate Sandstone outcrops in Lisbon Valley, Utah. The Spectral Angle Mapper method was applied on Hymap hyperspectral data to classify the extent of altered and unaltered outcrops, as well as to map the changes in mineral content within the outcrops. The Spectral Feature Fitting method was used to identify lithological changes in the area. Reflectance spectroscopy, thin section studies, major, minor, and trace element analyses, and stable carbon and oxygen studies on both bleached (altered) and unbleached (unaltered) samples were successfully used to delineate areas of similar rock composition and relate changes due to hydrocarbons leaking from underlying petroleum reservoirs. Unbleached Wingate Sandstone samples had higher hematite and feldspar content than bleached Wingate samples, which were characterized by larger amounts of clay, calcite, and pyrite. Some bleached samples also had higher concentrations of elements (U. Mo) characteristic of hydrocarbon-related reducing environments, and were depleted in C-13 when compared to the unbleached samples. Based on these results, the following model of chemical reactions is suggested for diagnostic changes within Wingate Sandstone. Hydrocarbon-induced reducing environment caused the transformation of sulfate ion (obtained from groundwater or from oxidation of H2S) to sulfide ion, resulting in the reduction of hematite to pyrite. The released hydrogen ion from this reaction reacted with available feldspars in the rock, leading to precipitation of kaolinite. These conditions favor the reaction between bicarbonate ion and Ca2+ ions that can be obtained from the groundwater, leading to precipitation of calcite in pore spaces left open after the reduction and removal of hematite. (C) 2012 Elsevier Ltd. All rights reserved.